Ok, I would assume when Gamages/Swan & Edgar/Constantin Weisz etc order their watches from China, they have to order a set amount, like a minimum quantity of watches to get the price they want, that's why most of their watches tend to be Limited editions, so when TJC put up a stock quantity, that's probably the max TJC will be receiving/can sell from that consignment of watches that Gamages will be getting from their supplier for any particular watch, so although they don't have in stock they will know the quantity that they will be ordering of that watch etc.
Although they claim to design their watches, in reality they are just picking different options from a catalogue watch parts. There will be a whole range of cases, dials, hands, movements, colours, finishes etc to choose from, bit like mix and match and then have their name and logo put on.